The commenter is likely referring to the original S&P 90, which mandated a certain number of stocks in different sectors. At the time those numbers were 50 industrials, 20 utilities, and 15 railroads. The breakdown shifted as the economy changed until the 80's when they did away with sector quotas in favor of rules closer to today (basing allocation on market cap).

Regardless, the S&P 500 also excludes a company like Microstrategy (the company that holds Bitcoin) from their index, had excluded Robinhood for a wile due to missing the profit requirements, and so on. It was never "meant" to cover the 500 largest companies by market cap, and has generally resisted pressure to change that.